1. Field of the Invention
The present invention relates to an improved inorganic gel-polymer electrolyte and to electrochemical cells employing such an electrolyte.
2. Description of the Prior Art
The electrolyte in commercially available lithium-ion electrochemical cells is typically a solution of a lithium salt, such as LiPF6 or LiBF4, in a mixture of carbonate solvents, such as ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or propylene carbonate (PC). Although these liquid electrolytes are attractive as they offer good lithium ion conductivity (10xe2x88x923 S/cm), they do present certain manufacturing and safety problems. These problems arise primarily from their low viscosity, which renders the electrolytes prone to leakage, and from the volatility and flammability of the organic components, which enable the electrolytes to readily burn and vent from the cells. In cells that utilize liquid electrolytes, a microporous separator film is used to electrically isolate the positive and negative electrodes. Since solid electrolyte materials do not leak and negate the requirement for a separator film, considerable cost savings are possible.
Currently available alternatives to liquid electrolytes include ceramic, solid polymer and organic gel-polymer electrolytes. Examples of ceramic materials include lithium phosphorous oxynitride. This ceramic material conducts lithium and can act as a separator, eliminating the need for a microporous separator film. However, its low conductivity, typically 10xe2x88x927 S/cm, which is over 1000 times less than liquid electrolytes and manufacturing limitations, has inhibited further development and application of this material. Solid polymer materials, such as polyethyleneoxide (PEO), and variants such as polyphenolates, offer commercially viable processing properties, and the potential for cost savings, but they must operate at elevated temperature, normally about 40xc2x0 C. to about 80xc2x0 C., precluding their use in many applications.
To obviate the low conductivity limitation of solid polymer materials, organic gel-polymer electrolytes have been developed. Gel-polymer materials are prepared by adding a solution of a lithium salt in an organic solvent to a solid organic polymer material. Materials using a variety of organic polymers, including (poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(ethylene oxide) (PEO), poly(phosphazene) (PPz), poly(vinyl chloride) (PVC) and polytriethylene glycol(dimethacrylate) (PTGEGD), have been investigated and employed in electrochemical cells. The organic solvent and salt enable the material to offer conductivity near that of liquid electrolytes, while the polymeric matrix provides a solid structure in which the liquid electrolyte is immobilized or absorbed. While these materials mitigate leakage problems encountered with liquid materials, they do not offer any improvement in flammability and, despite considerable research and expense, they have eluded commercialization because of manufacturing, safety and scale-up problems. In the gel-polymer technology where PVDF-HFP is used as the polymer, after lamination of cell stack, a plasticizer, typically dibutylpthalate (DBP), is removed from the polymer before the electrolyte (such as EC:DMC, LiPF6) is infused into the electrode stack. Some of the challenges associated with implementing this technology relate to scaling the extraction of DBP and infusion of the liquid electrolyte on a commercial scale.
In an article entitled xe2x80x9cSynthesis and Properties of Sol-Gel Derived Electrodes and Electrolyte Materialsxe2x80x9d by J Harreld et al. appearing in xe2x80x9cThe Proceedings of the 5th Workshop for Battery Exploratory Developmentxe2x80x9d, published on Jun. 30, 1997, there is disclosed a solid electrolyte material which exhibits a high lithium-ion conductivity. The solid electrolyte was prepared by a known sol-gel process wherein a hydrolyzed silica precursor, namely, (tetramethyl) orthosilicate, Si(OCH3)4, was admixed with a lithium ion conducting liquid electrolyte along with deionized water and an acid catalyst to form a lithium conductive sol. The liquid lithium electrolyte was prepared by dissolving ethylene carbonate with lithium borofluorate, LiBF4 in a propylene carbonate solvent to a molarity of 1.65M. After ageing and drying, Sixe2x80x94Oxe2x80x94Si linkages form within the sol and a three-dimensional silicate network develops in which the liquid phase is encapsulated. The liquid electrolyte provides ionic conductivity while the silica linkages support the liquid electrolyte.
Experimentation with the solid electrolyte material disclosed in the above article has shown that the lithium borofluorate, LiBF4, component in the liquid electrolyte reacts with water in the reaction mixture and is not stable. This of course precludes use of this solid electrolyte material in the fabrication of a working electrochemical cell.
A related material, using a water stable salt, was successfully demonstrated in a Li-ion cell as described in the above referred to co-pending application Ser. No. 09/137,492, filed on Aug. 21, 1998, the disclosure of which is incorporated herein by reference. These inorganic gel-polymer electrolytes are also based on the immobilization of a liquid electrolyte in an inorganic silica network. The conductivity of the dried gels is comparable to that of a liquid electrolyte, 3.5xc3x9710xe2x88x923 S/cm at 25xc2x0 C., (EC: PC, LiBF4). At 80xc2x0 C. the conductivity approached 10xe2x88x922 S/cm. The sol-gel process used involved the hydrolysis of a silicon alkoxide (typically silicon tetraethoxysilane (TEOS), in an aqueous solution. Although the amount of water added was limited to that required for the hydrolysis reaction, residual water accelerated capacity fade when these electrolytes were used in Li-ion (LiCoO2/C) cells. All other characteristics of the cells evaluated were comparable to cells prepared with non-aqueous electrolytes. Prior efforts demonstrated that inorganic gel-polymer electrolytes of this type may be used in Li-ion cells, but to improve capacity fade the water must be eliminated.
In accordance with the present invention an anhydrous inorganic gel-polymer electrolyte material for use in electrochemical cells, and particular lithium-ion cells, is made by a non-aqueous sol-gel process. The anhydrous inorganic gel-polymer electrolyte is prepared by reacting a metal halide, preferably a metal chloride, such as silicon tetrachloride, with an alcohol, such as tert-butyl alcohol, in an anhydrous organic solvent. An active metal-ion conducting liquid electrolyte, preferably a lithium-ion electrolyte, is encapsulated into the resulting three-dimensional inorganic metal oxide polymer network. The liquid electrolyte yields good conductivity while the metal matrix imparts mechanical and thermal stability to the material. In contrast to the above described prior art, the anhydrous inorganic gel polymer electrolyte of the invention is compatible with the lithium-ion cell chemistry since it does not contain water. In addition, the inorganic gel-polymer is able to function as both an electrolyte and a separator in a lithium-ion cell.
The liquid electrolyte may be prepared by dissolving an alkali metal or alkaline earth metal salt with a suitable solvent, such as an organic solvent containing one or more carbonates. The admixture is added as a liquid precursor to an electrochemical cell employing the usual anode and cathode materials and is allowed to gel or polymerize in situ to form the inorganic solid gel-polymer electrolyte of the invention.
Thus, the invention also comprehends a novel method for fabricating an electrochemical cell employing a sol-gel electrolyte in which the electrolyte is added to the cell in its unactivated condition as a liquid and then solidifies to form a solid electrolyte, thereby enabling the manufacture of solid electrochemical cells as a liquid electrochemical system.
The lithium containing salt used in the inorganic gel polymer electrolyte of the invention is preferably an alkali metal or alkaline earth metal salt which is non-reactive with water including any one of the following organic lithium salts:
(a) Lithium Bisperfluoroethanesulfonimide (LiN(SO2C2F5)2)
(b) Lithium Bis[5-fluoro-2-olato-1-benzenesulfonato (2-)-O,O]borate(1-) (LiB(C6H3FO(SO3))2)
(c) Lithium bis-(trifluoromethanesulfonyl)imide (LiN (SO2CF3)2)
(d) Lithium tris-(trifluoromethanesulfonyl)-methide (LiC(SO2CF3)3)
(e) Lithium bis-(trifluoromethylsulfonyl)methane (LiCH(SO2CF3)2 
The preferred lithium salt for use in the inorganic gel-polymer electrolyte of the invention is lithium Bisperfluoroethanesulfonimide (LiN(SO2C2F5)2).
The solvent used to prepare the metal-ion conducting electrolyte according to the invention is preferably a solvent selected from the group consisting of ethylene carbonate, dimethyl carbonate, diethylcarbonate, ethyl methyl carbonate, methyl propylcarbonate and dimethylether as well as mixtures thereof.
The metal halide used to prepare the anhydrous inorganic gel-polymer electrolyte of the invention is preferably silicon tetrachloride (SiCl4) but halides of other metals such as aluminum, titanium, vanadium, chromium, molybdenum, and tungsten may also be useful in the sol-gel process.
In addition, other alcohols besides tert-butyl alcohol may be used in the preparation of the inorganic gel-polymer electrolyte of the invention as will readily occur to those skilled in the art.
The electrochemical cell according to the invention may employ any conventional anode material but preferably the anode is carbon. Other anode materials that may be used include lithium, titanium oxide, tin oxide, tin or germanium oxide, for example. Similarly, the cathode material may be any conventional cathode material but preferably is a lithiated transition metal oxide such as lithium cobalt oxide LiCoO2. Other suitable cathode materials include LiNiO2, LiNi0.82Co0.18O2 and LiMn2O4, for example.